The penetrative ability of x-rays makes them valuable for structural imaging applications ranging from medical imaging and materials research to quality control and security. Transmission imaging with x-rays is typically used to produce an attenuation contrast image of the material of interest. However, attenuation contrast images suffer from poor contrast sensitivity that poses severe limitations in many applications. The contrast sensitivity of transmission imaging is also an issue in medical imaging. Detection of early stage cancer (as represented by tumors smaller than 1 cm) is of vital importance. Transmission imaging is a workhorse imaging modality, yet is not sensitive enough to reliably detect such tumors.